There are certain situations in which a fluorescent ink is used for detection purposes. Such fluorescent inks will have one color, such as black or red, when viewed in ambient light, but upon being exposed to ultraviolet light will display another color such as green or yellow. Examples in which such inks can be used are in the machine processing of various types of information on tickets, tags, labels, postage indicia or imprints, and the like. In the case of postage indicia, facer canceller tables and metered mail handling equipment are fitted with luminescent ink detecting devices that expose the postage indicia to an ultraviolet light during the automatic processing of mail to expose the indicia to an ultraviolet light during the automatic processing of mail to assure the indicia is genuine and postage has been paid. If the specified luminescent signal is not viewed during exposure, the postal clerk is required to hand cancel the postage indicia if it proves genuine.
Fluorescent inks that have been used in postage meters to print postage indicia in the past have worked well, but they have certain shortcomings. The compositions of these fluorescent inks has been in the form of a dispersion. Postage meter inks have been made by dispersing a fluorescent pigment (toner) into an oily liquid. Other pigments have been added to provide color shifts. These inks, however, do not meet all the contemporary requirements.
In fact, dispersion inks have drawbacks due to the nature of the ink itself. As a dispersion, great care must be taken during formulation to provide a uniform ink. The particle size distribution of the toner pigments must be controlled as well as any added coloring pigments. As the experienced formulator will attest, matching these pigment particle sizes to prevent settling and reduce the tendency to aggregate, agglomerate, and settle is quite difficult. For practical purposes, one must require some agitation prior to use. Thus, a dispersion ink must rely on an operator to "Shake Well Before Use" in order to provide a consistently acceptable quality of print. This aggregation and agglomeration also requires the use of large pore foams for ink delivery. As in any process which transfers ink, a smooth layer of ink is required for a consistently uniform print with few voids and intensity variations throughout the inked regions. This is accomplished by the use of a smooth, tight pore inking surface with average pore dimensions usually on the order of 5 to 15 micrometers. At this level, agglomerated pigments tend to plug these openings causing reduced quality of print.
Another problem which may be attributed to the nature of a pigmented postage meter ink is print to print color variation. When multiple pigments are used, the particle size distribution and specific gravity of the pigments do not settle in a uniform manner. Thus, the portion of ink near the surface may be a significantly different color than the ink near the bottom. This difference is observed as inconsistent colors from print to print.
It has long been recognized that a fluorescent ink in solution form would be more desirable because of its advantageous properties of Newtonian viscosities that are not dependent on shear rates under normal conditions. Thus, the flow properties of solutions can be controlled more easily than dispersions. To date, no adequate solution fluorescent ink has been found because certain properties are required that could not be achieved in the past. A boiling point of over 250.degree. C. along with a very low vapor pressure, such as less than 0.05 mm Hg (at 25.degree. C.), are criteria that provide solvents which do not dry on critical machine components and have a reduced hazard from fire or explosion. This safety aspect of the solvents is a critical criteria. To ensure safety to the machine user, the solvents should have been tested and ruled acceptable.